I just realized I sent you to the powerstore features and not the ecart features. Please view the updated link from my previous post.
eCart 5 replaces eCart 4, so they cannot both be installed at the same time.
It is backward compatible because it won't change any code by default until you enter an interface, it will recognize all of the functionality applied from eCart 4, and when you apply from an interface it will automatically update the code to eCart 5 code.